You may have heard that some developers have gotten their hands on Windows Mobile 6.1 for the Palm Treo 700wx. If you want to learn more, then take a look at this forum on XDA-Developers:
Flashing a phone's ROM is not for beginners. This is true especially when you're loading a custom ROM. If you make a mistake, then your phone may turn into a permanent brick that you can use as a paperweight. I always advise people to ask for assistance or to hand the phone over to an expert who can do the flashing correctly.
In my opinion, there are significant advantages to running Windows Mobile 6.1 over Windows Mobile 6.0. That change from ".0" to ".1" at the end of 6 makes a big difference. Now we have Windows Mobile 6.5 but that's not available for the Treo (at least not now). Not sure about the version differences? Take a look at this chart that provides an overview of Windows Mobile 5.0, 6.0, and 6.1: